For example, an uninterruptible power supply device (UPS: Uninterruptible Power Supply) is a power supply device that normally charges a storage battery by a commercial AC power supply and upon power outage, converts the power stored in the storage battery to an AC power and outputs the AC power.
Such an uninterruptible power supply device has, for example, a configuration in which the same bidirectional inverter is used in common for both charging and discharging of the storage battery (see, for example, Patent Literature 1). In this case, in charging of the storage battery, the bidirectional inverter operates as a converter for performing AC-to-DC conversion, and in discharging of the storage battery, the bidirectional inverter operates as an inverter for performing DC-to-AC conversion. An AC switch is provided for disconnecting the bidirectional inverter and the commercial power supply from each other in discharging of the storage battery.
Such an AC switch is configured from, for example, a relay contact and a semiconductor switch connected in parallel to each other. The relay contact has an advantage of having a smaller conduction resistance than the semiconductor switch, but the time from when a turn-on command is issued (excitation) until the contact is actually closed is longer than in a case of the semiconductor switch. In addition, the relay contact is not suitable for highly frequent current ON/OFF operation. On the other hand, the semiconductor switch has a turn-on speed extremely faster than that of the relay contact, and is suitable even for highly frequent current ON/OFF operation.
Therefore, using the AC switch having the relay contact and the semiconductor switch connected in parallel to each other enables ON/OFF operation that includes both advantages. For example, at the time of starting the uninterruptible power supply device in a case where the commercial power supply is normal, the following operation can be performed in which the semiconductor switch is closed first, then the relay contact is closed, and thereafter the semiconductor switch is opened (see paragraph [0036] in Patent Literature 1). Thus, quick closing by the semiconductor switch at the initial stage is achieved, the relay contact to be closed later is not subjected to stress at the time of closing, and after both are closed, only the relay contact is kept closed, whereby the conduction resistance is reduced.